It appears that many on here have various problems with the 5GHz band on Draytek products, so this is only an account of my experience.

Briefly, I have a 2925ac to which all of my wifi devices connect quite happily on both 2.4 GHz and 5Ghz. I am using its internal RADIUS server for 802.1x on my main SSID, two other SSIDs use plain WPA2.

I also have an AP902 (have used 2 of these and an AP900, all with same problem) connected by ethernet to the 2925ac. SSIDs on the 2.4GHz and 5GHz bands are set with exactly the same settings on the AP902 and yet I found that a) a Surface Pro 4 would connect on the 2.4GHz band but resolutely refused to connect on the 5GHz band, and b) iOS devices might connect on 5GHz if I was lucky but only at vastly reduced internet speeds (40M down to 1.5M for example) and they could not browse the LAN. On 2.4GHz they had no problems at all. On my iPhone X the 5GHz connection would continually connect and then drop, even when standing next to the AP.

I have always used the APs by switching off DHCP and giving them IP addresses in their GUI - e.g. 192.168.1.2 etc

On a whim yesterday, I decided to try using IP binding in the 2925ac and switching on DHCP in the AP to give the AP902 the same fixed IP address that it has always had.

Hey Presto! as if by magic the Surface Pro 4 connected instantly on 5GHz and all iOS devices connect at full speed.

I have no idea why this should have made a difference, nor why 5GHz should differ from 2.4GHz with exactly the same settings, but it seems to have solved my problem. Hopefully this may help someone else who is about to chuck their Draytek gear in the river, as many threaten to do in their frustration.

Thought I'd post an update on this as things went downhill after my initial euphoria as noted above. After a couple of days the same old problems resurfaced when 5Ghz was activated on the AP902.

So I just left it off and resigned myself to it never working properly. In an idle moment last week I thought I'd have another go - so I activated 5GHz again but this time I used 11n only - not using the ac facility at all.

Hey presto! - all iOS devices and the Surface connect and don't drop speed after a short while. Been up for about a week and all seems well.

Pity about the ac but n is better than nothing I guess. Bizarre that ac on the 2925ac works fine with all these devices, but on the AP902 it doesn't.

It could depend on placement and interference if you've seen 11n mode improve things. My reason for that is, it moves the channel bandwidth down to 40MHz from 11ac's 80MHz. You could test the channel bandwidth of 40MHz with 11ac in the [WLAN 5GHz] > [Advanced Setting] section.

Are you using the non-DFS channels of 36-48 or one of the DFS channel groups? (52-64 & 100-140) It might be worth checking Syslog if you are using the DFS channels, to see if the AP's DFS checking is interrupting your wireless connections.

I'm pretty sure that over the months I tried restricting the bandwidth to 20/40MHz and it didn't help, but I'll give it another go. I've also tried various channel settings although I can't remember which ones now. It's presently set on Auto and is using channel 124 (on the 11n) with no apparent problems*. Placement is about 2m line of sight so I pretty sure it wasn't that. The router itself is using channel 100 with bandwidth 20/40/80 and never has a problem.

* I've just changed to 11a+11n+11ac to test bandwidth setting and channel has switched to 136

Well so far so good - changed the AP902 bandwidth on 5GHz a+n+ac to Auto 20/40 and all devices are still connecting quite happily. Could this be it?

I have a similar problem. I added an AP902 to my 2860ac to extend coverage into the garden, this was after trying various lash ups using old routers from Fritzbox and BT. Worked okay with one AP902 and the 2860, but adding another AP902 caused the 5Ghz band to drop speed to anything from 1mps down to nothing, this happens on Android, iPhnone, Windowa Linux and Mac OSX. I initially thought there was a problem with the new (used) AP902 but swapping them over confirmed both AP902s are the same. Switching 5GHz WLAN off and on again will often bring it back on that device, but it will then vanish on the other one, I seldom have both working.
curiously upload speed from teh device is fine and runs flat out. I've tried pushing profiles from the 2860 using central management, configuring by hand, moving away from the DFS bands, re-siting the AP902s and dropping down to 20/40 MHz bandwidth, nothing has worked.
If anyone has any ideas I'd love to get this to work, I'm now out of ideas though.